vacuum options charcoal canister with twin dcnfs

Kevin Cozzo

Kevin Cozzo

True Classic
okay, I am pre planning my plumbing before I start putting on the dual carbs, and I did a search on the carcoal canister, because I would like to smell the least amount of gas fumes possible. Mine is a 77 with the round can, and presently I have a vacuum line from the carb hooked up, and a vent line from the carb bowl, and one from the gas tank. Now I know all I can vent is the gas tank, but I'm figuring for the canister to work somewhat is to have a vacuum source. The new intake has no ports, but the carbs each have a post and pre throttle plate vacuum source...I'm guessing I would want to hook up to the vacuum source that is a constant, or pre throttle plate, so there is vacuum all the time, but what I'm wondering is since there are 2 separate carbs, and intake runners, if I hook up to one of those, will it "imbalance" the carburetion....I noticed that the charcoal filter vacuum line flows pretty easily when I apply suction...so it would seem that one carb would be getting a lot of extra air, unlike hooking up a distributor advance, where there is resistance on the vacuum being drawn.
should just run the gas vent line into the canister, and leave it at that? I thought maybe also, connecting both vacuums together, one from each carb, and then to the canister?
Thoughts anyone? I couldn't find this on a search
 
Well glad to know I'm not alone, obviously, everyone here is just living with gas fumes- haha....I have done a ton of google searches on this, most interesting was on a vw bus forum, where one guy wants to know the same thing- tank venting with dual carbs, and one guy on the forum, tirelessly telling the entire group it's just not possible with open air filters. I have noticed that old GM evap systems, worked off a vacuum operated solenoid, and another vacuum line to the pvc valve (which is full manifold vacuum, I guess)- it was triggered post throttle, so it only purged above idle, using manifold vacuum.
Another guy was trying to tune his aftermarket carb, with aftermarket manifold, on some sorta GM, and the Evap system greatly affected his carb tuning, as he had capped it off, and replaced it after tuning- big vacuum leak- he was told that the original Vac line to the canister, was "metered" with a very small restriction in the stock manifold barb going to canister
I have decided that I'm going to make an airbox, going to a remote filter, and put some kind of tube in the pathway of airflow to create some sorta light vacuum, as the vacuum line flows very freely- should be as simple as tapping a nipple in the side of the inlet tube on the box,
and not having to worry about it messing with carb tuning
 
I am in the a boat too, not the same boat but one very similar. I have a '78 with the large charcoal canister. I switched carbs to the 36 DCNVA and do not think I have a vacuum port on my carb that is available. I might be able to drill and tap one though but hesitant to try that.

I'll be following this thread closely.
 
Is there a vacuum port on the intake manifold that you could use (with a restrictor of sorts in line)? Perhaps one intended for a vacuum brake booster?
 
On my intake- no, as it has split runners also because it is a dual carb manifold...on my carbs they both have vacuum ports that are on the manifold side....Previously, I had the canister hooked up to the same port, but I am wondering if I hook it up on just one carb- will it "imbalance" the carbs? Wonder if I tee the two manifold vacuum ports together if it will equal out the vacuum loss...interestingly enough, I found an article on another vw forum about putting a "balancing tube" between split intakes when using a vac advance distributor to stop "pulsing" of the vacuum signal
https://www.thesamba.com/vw/forum/archive/index.php/o-t--t-615627--.html
 
Dual carb manifolds for IDFs on spiders and DCNFs for 128s with brake boosters have a vacuum port off just one of the four runners but the brake booster is a closed circuit so all the manifold does is provide a vacuum, not an actual flow of air.

Are you trying to draw the fumes from the charcoal cannister into the carb(s) when the engine is running or just for gathering gas fumes from the tank?
All three of my Fiats run a simple vent hose from the tank with a cap on the end and a tiny hole in the cap, same idea as guys who used to drill vent holes in their gas caps. I get no fumes and even when parked in the garage there are no noticeable fumes. Are there some fumes escaping into the atmosphere, probably but also probably not more than my lawn mower. Besides, the head of the federal government says we have no problem polluting the air so as a retired federal employee, who am I to argue with such intelligent men in high places.

Sarcasm content in the above post is possible...
 
just looking to purge the canister, which will vent the gas tank. I know it's not much, but it gets real hot in my garage in the summer, and if I can do this with just some vacuum lines, and maybe a plastic purge valve, then no harm no foul... I am thinking I am just going to mount a small tube in the airbox, in the flow of air, and get the venturi effect to lightly draw air from the charcoal canister, as there is hardly any restriction on that vacuum line. That way I don't have to run off any manifold vacuum that will affect carb tuning
 
Interesting thoughts tho about the info I found on "balance tubes" between the independent manifold plenums tho...I heard it makes balancing the carbs easier...guess I will find out as my carbs have manifold side vacuum ports
 
AKimball92 said:
I might be able to drill and tap one though but hesitant to try that
Click to expand...
yeah, know what you mean...have done things like that on my DATR carb...for my carbon canister, if I remember correctly. I found pics or drawings of one with the vacuum port to be sure...stole the nipple out of a stock 79 carb, the useless one with vac secondaries...glued it in with epoxy. As someone told me "you can always JB weld over the hole if you f*** it up, lol...unless you drill into a secret passage, lol. I haven't checked my old carb (the Datr that I mentioned) I had the evap canister hooked up, and the car ran fine, but always had warm start problems. Had to crack the throttle to start it once it was warmed up
 
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